Jacob Gould Schurman Professor of Applied Mathematics at Cornell University

Several newspapers have called me to ask how many handshakes separate us from someone who worked in the WTC. Using the sociologists' best estimate of about 300 acquaintances per person in the US, and assuming no overlap in anyone's friendship circles (very crude but no one knows how to do better), I estimate that about 1 in 20 of us knows someone who worked in WTC. If you go one more step, essentially everyone in the US is just two handshakes away. And two degrees of separation feels intimate — we know what friend of a friend means, intuitively — whereas three is much more nebulous. In other words, there's a psychological gulf, a transition, right between 2 and 3, and we are all on the intimate side of that gulf. I think that has something to do with why the horror feels so close.